Wash the potatoes and put them to boil in water with 1 tablespoon of vinegar, but let them remain semi-softened. Then cool them and cut them into oblong circles. I used a grater. Grease a tray and arrange the potatoes, sprinkle with some of the spices, crumbled feta cheese and a little oil.
Then arrange a second layer of potatoes, feta cheese and spices. Next, arrange a layer of tomatoes, olives, spices, feta cheese, and finish off with a layer of potatoes and some feta cheese.
Make a béchamel sauce from the flour that has been fried in 3 1/3 tbsp (50 ml) of oil / butter. Dilute it with the milk while stirring and stir some more until the sauce thickens. Beat the eggs with the yoghurt and baking soda, and add them to the béchamel sauce while stirring continuously.
Top the gratin with this mixture and put it to bake for about half an hour in a moderately heated oven.